This invention relates to a fish rearing system having a water circulation system for circulating the rearing water in a fish rearing water tank, and, particularly to a fish rearing system capable of performing water quality control at a low cost by a rational combination of devices.
Conventionally known fish rearing system is generally comprising a water circulation system for discharging the rearing water from a discharge port provided at the bottom of a rearing water tank and re-supplying the discharged water to the rearing water tank.
Since the fish rearing system of this type is designed to rear fish in the rearing water tank, a special consideration is given to contamination of the water caused by the metabolism of fish. Thus, the heretofore proposed art has concentrated on how efficiently the water contamination can be removed within a limited space of the rearing water tank, the water circulation system and the like.
In order to improve the efficiency of the fish rearing system by removing the water contamination and regulating the rearing environment, the fish rearing system generally includes a filter device for capturing residual feed and fish feces contained in the water, an aeration device for supplying oxygen to the rearing water, an ammonium treating device for treating ammonium generated during rearing, or an ozone supplying section.
In performing water quality control of the fish rearing system, it is particularly important to control contents of salt, mineral, residual feed and fish feces, protein and ammonium, and to treat nitrogen dioxide, nitrogen dioxide, carbon oxide and oxygen which are generated during decomposition process of the above-mentioned substances. For example, it is known that ammoniac nitrogen is harmful to fish. It is observed that when the ammoniac nitrogen content in the rearing water exceeds 3 ppm, certain kinds of fish lose their eating activity sharply. Although it is known that a content of nitrogen nitrate generated during nitrifyingly decomposition of ammoniac nitrogen is not so harmful to fish, an acceptable tolerance for fish rearing is limited to the range of from 50 ppm to 500 ppm, though the tolerance depends on different kinds of fish.
Fish feces, an organic substance such as protein, and ammonium produced by a biological metabolism of fish and residual feed are particularly giving a great effect to increase water contamination, and these ill-effect elements are removed by various treatment device, or purified by the metabolism function of microorganisms which are present in the water circulation path. For an index of water contamination, the term of xe2x80x9cbiochemical oxygen demand (hereinafter referred to as xe2x80x9cBODxe2x80x9d)xe2x80x9d is generally used. The BOD represents an amount of oxygen (mg/l) dissolved and contained in the water which is consumed by proliferation activity or breathing function of bacteria which are present in the water.
For example, a nitrogen compound (ammoniac nitrogen, nitrite-nitrogen, nitrate-nitrogen), which is generated by the metabolism of fish and which is one of elements to be limited in biological production, is purified by utilizing the metabolism function of, so-called denitrification bacteria and nitrification bacteria, such as, aerobic microorganisms or anaerobic microorganisms present in the water circulation path. Ammoniac nitrogen (NH4xe2x80x94N) is biologically oxidized by nitrification of aerobic microorganisms and turned into nitrite-nitrogen or nitrate-nitrogen (NO2xe2x80x94N, or NO3xe2x80x94N), and biologically reduced further to nitrogen-oxide gas (NO2) or nitrogen gas (N2) by anaerobic bacteria. During this process, anaerobic bacteria require the presence of an organic carbon source serving as a hydrogen supply body for acquiring the energy required for biological reduction.
In the case where the metabolic function of bacteria is utilized, it is necessary that a physiological activities should be taken into consideration so that the metabolism of microorganisms can be effected under the suitable condition.
For example, in nitrification of ammoniac nitrogen (NH4xe2x80x94N), a nitrification rate reaches to about 100% when a BOD load is low, but the nitrification rate is rapidly lowered when the BOD load approaches a certain value. Further, it is known that, in denitrification of nitrite-nitrogen or nitrate-nitrogen (NO2xe2x80x94N or NO3xe2x80x94N), the higher the BOD load, the faster the denitrification speed. In other words, a value of BOD load suitable for nitrification action and a value of BOD for denitrification are the conditions contrary to each other.
Further, in this type of fish rearing system, if, for example, removal of water contaminants and regulation of the rearing environment can be effected simultaneously by a single treatment section, such a system is desirable for saving energy of the rearing system. Also, it is necessary for each treatment device to make a correction or complementary treatment of the treatment made by the other treatment section.
In view of the above-described situation, it is an object of the present invention to provide the fish rearing system capable of performing water quality control at a low cost by a rational combination of devices.
This invention relates to a fish rearing system having a water circulation system for discharging the rearing water from a discharge port provided at the bottom of a rearing water tank and re-supplying the discharged rearing water to the rearing water tank, characterized in that the water circulation system comprising: a trapping device for capturing residual feed and fish feces contained in the rearing water, a residual feed detecting sensor for detecting the residual feed contained in the rearing water, a denitrification device for denitrifies a nitrogen compound contained in the rearing water, a bubble surfacing device for generating bubbles by supplying air and/or ozone and removing surface active substances by capturing the generated bubbles, an ultraviolet disinfection device for disinfecting the circulating rearing water, an ammonium treating section for decomposing ammonium contained in the rearing water, and an aeration section for dissolving oxygen into the rearing water, and wherein the denitrification device is disposed at the upstream of the bubble surfacing device and in a circulation path branched out from a main circulation path.
Before effecting any/all water quality controls, it is required to provide the trapping device for capturing residual feed and fish feces contained in the water. Detection of residual feed contained in the water allows to make an adjustment of amount of feed to increase or decrease thereof. The denitrification device is required to decompose nitrite-nitrogen, which is the final product of ammonium, into nitrogen gas. The supply of air is not only desirable for regulation of the rearing environment, but the supply of air and/or ozone is effective in that it generates bubbles, and organic substances such as fatty acid and protein adhered to bubbles can be removed by capturing bubbles. The ultraviolet disinfection device disinfects germs contained in the water which cause illness to fish. This device also decomposes ozone contained in the rearing water by irradiation of ultraviolet rays. Further, the ammonium treatment device is required for performing a purification treatment of ammonium generated by physiological activities of fish and which is one of elements to cause water contamination. The aeration device is for dissolving oxygen in the water which is vital to keep the fish alive.
By disposing the denitrification device at the upstream of the bubble surfacing device as described above, an organic carbon source, which is necessary for denitrification by anaerobic bacteria, can be provided by organic substances contained in the circulating water. Therefore, this can save time and labor of adding additives to supply the organic substances required for denitrification, or even if such adding of the additives is required, it is sufficient to add a small amount of additives. This allows to effect the denitrification treatment efficiently and less costly, which, in turn, allows to perform the quality control of the water efficiently. Further, by disposing the denitrification device in the branched water circulation passage, reducing the amount of water flowing through the branched water circulating passage, and prolonging the time for the water residing in the denitrification device, the denitrification can be achieved to a satisfactory level. That is, this causes the lack of the absolute amount of dissolved oxygen in the denitrification device so that the inside thereof easily becomes the anaerobic state, in which the anaerobic bacteria become active to effect denitrification more efficiently.
Further, with the present invention, the above-described ammonium treatment device is disposed at the downstream of the bubble surfacing device, and comprising two treatment device of a immersion treatment device and a non-immersion treatment section.
By disposing the ammonium treatment section at the downstream of the bubble surfacing device, the rearing water from which protein and the like have been removed is communicated with the ammonium treatment section. Thus, the load in the ammonium treatment device is reduced, and decomposition of ammonium is promoted. Further, the ammonium treatment section comprises two treatment sections, one of which is the non-immersion treatment device where a contact area with the air is larger, thereby to facilitate the use of oxygen contained in the air. This allows to reduce an amount of use of an expensive oxygen, and, at the same time, a high content of ammonium in the rearing water flowing from the upstream can be decomposed to a certain degree first in the non-immersion treatment device, then decomposed to a further low content in the immersion treatment device. Thus, it is possible to improve the efficiency of ammonium decomposition than that of a one-stage ammonium treatment.
Moreover, with the present invention, the trapping device is comprising a particle trap provided at the bottom of the rearing water tank and in which the sediment containing residual feed and fish feces is accumulated; a sediment trap for separating the sediment, which has flown in through the circulating passage branched from the particle trap, from the water; and a filter section for catching minute residual feed and other residual feed which have escaped the particle trap and the sediment trap and are still contained in the water. In addition, the residual feed detecting sensor is an ultrasonic sensor which is disposed at the upstream of the sediment trap and interposed in the water circulating passage branched from the particle trap and communicating with the sediment trap.
As described above, with the present invention, the sediment including residual feed and fish feces can be caught by the particle trap, the sediment trap and the filter device, and the solid substances can be removed from the rearing water, thereby the rearing water in a clean condition can be circulated. Further, a specific ultrasonic sensor is used for the residual feed sensor, so that the residual feed can be easily detected from the sediment which is a muddy mixture of residual feed and fish feces. A detection result of the residual feed is provided to an external computer, so that the feeding rate can be easily controlled. By disposing the residual feed detection sensor in the circulating passage branched from the particle trap, a quantity of the flow of the water flowing through this branched circulating passage is reduced, and a flow velocity may be delayed by providing a water retention section and the like in the circulating passage. Then, the residual feed contained in the water, which flows slowly and in a small quantity of flow, can be determined easily, thereby to improve the detection accuracy of the residual feed detection sensor.
Moreover, with the fish rearing system of the present invention, the denitrification device and the immersion treatment device each includes a fluidized bed filter, the non-immersion treatment device includes a rotary bio-contactor tank, and the aeration device includes a pressure dissolving device or a hollow fiber membrane module.
The fluidized bed filter used in the denitrification device and the immersion treatment device contains therein bacteria, and the ammonium contained in the rearing water is decomposed by the bacteria. Further, by providing the rotary bio-contactor tank to the non-immersion treatment device, the area for contacting with the air is made larger so as to use the oxygen contained in the air effectively for conducting the treatment. Also, by providing the pressure dissolving device or the hollow fiber membrane module in the aeration device, the oxygen is supplied through fine pores of the hollow fiber membrane, specifically, the oxygen is jet out from the fine pores, so that the oxygen can be efficiently dissolved in the water. Thus, it lowers the cost than the use of liquid oxygen.
Moreover, in the fish rearing system of the present invention, the ultraviolet disinfection device is disposed at the downstream of the bubble surfacing device.
With such arrangement of disposing the ultraviolet disinfection device at the downstream of the bubble surfacing device, germs contained in the rearing water which cause illness to fish can be subjected to disinfection treatment. Further, in the bubble surfacing device, the supplied ozone can be decomposed. It is not desirable to have the ozone in high concentration dissolved in the water, because the aerobic bacteria which make the nitrification in the ammonium treatment section are killed, and the presence of high concentration of ozone is also harmful to fish. However, the ozone can be decomposed by the ultraviolet disinfection device, thereby to allow regulation of an amount of ozone.
As described above, the fish rearing system of the present invention is specifically structured by taking into consideration the opposing BOD load conditions in the nitrification treatment and denitrification treatment. Specifically, the nitrification is performed in the ammonium treatment device provided in the main water circulation passage at the downstream of the bubble surfacing device, and the denitrification is performed in the denitrification device interposed in the branched water circulating passage at the upstream of the bubble surfacing device with respect to the above-described main water flow passage.
With the present invention, the trapping device for catching residual feed and fish feces and the bubble surfacing device are provided at the upstream of the ammonium treatment section. Thus, by utilizing both the trapping section and the bubble surfacing device, the solid organic substances and organic substances dissolved in the water can be removed to the order of 90%, and nitrification in the ammonium treatment device, which is adapted to remove ammoniac nitrogen, can be effected more effectively. Specifically, in the ammonium treatment device provided in the main water circulation passage, the organic substances (residual feed, fish feces, protein which has been physiologically excreted by fish, and the like) which are the major factor of the BOD load can be removed to the utmost extent by a physical treatment prior to nitrification treatment, so that the nitrification can be performed effectively. Also, in the denitrification device provided in the branched water circulating passage at the upstream of the bubble surfacing device, the denitrification treatment can be conduced effectively with the water from which the organic substances have not been removed, namely, with the organic substances present in the water, without adding additional organic substances, or even when such adding of organic substances is required, a small amount of addition of organic substances is sufficient. The inside of denitrification device, where the denitrification treatment effected by anaerobic bacteria, is required to be kept in the anaerobic condition. However, when the full quantity of flow, or a flow close to the full quantity of flow, of the rearing water circulating with the dissolved residual oxygen in a relatively high concentration is communicated with the denitrification device, the inside of the denitrification device becomes the aerobic condition due to the oxygen contained in the water and the activity of anaerobic bacteria is obstructed. To avoid this, with the fish rearing system of the present invention, the denitrification device is provided in the water circulation passage branched from the main circulation passage so as to lower the quantity of flow of rearing water for flowing through the denitrification device. Thus, the anaerobic condition of the inside of the denitrification device can be maintained due to the lack of the absolute oxygen amount, which, in turn, promotes the activity of bacteria, resulting in purification of the rearing water.
Since the high purity oxygen is expensive, the utilization of oxygen contained in the air effectively at each treatment section leads to reduction of the costs. Specifically, in the ammonium treatment section where the ammoniac nitrogen is treated, not only the ammoniac nitrogen is treated, but also organic substances which are inevitably contained in the rearing water can be oxidized. To deal with this situation, the non-immersion treatment device having a larger area to contact with the air is disposed at the upstream where the BOD load becomes higher due to organic substances unavoidably contained in the rearing water, so as to use the oxygen contained in the air to the greatest extent to perform nitrification treatment and removal of organic substances. Consequently, the amount of consumption of expensive oxygen is reduced, thereby the costs are lowered. In addition, the immersion treatment device is disposed at the downstream of the non-immersion treatment device, where the contact ratio with bacteria is increased and the concentration of ammoniac nitrogen is lowered.
As described above, according to the present invention, the fish rearing system capable of performing the quality control of the water at a lower cost can be provided by a streamlined combination of various installations.